The present invention relates to a method of manufacturing a high heat flux regenerative circuit comprising a structure having an inner functional surface in contact with a first fluid and a set of channels formed in the body of the structure for conveying a second fluid in heat exchange relationship with the first fluid, the method consisting in using thermal spraying and machining operations around a reusable support core to build up the structure from said inner functional surface.
The invention also relates to a high heat flux regenerative circuit obtained by the method, such as the rocket engine combustion chamber.
Structures constituting high heat flux regenerative circuits are used in various contexts, for example in heat exchangers, in turbine blades cooled by a circulating liquid, or in the walls of combustion enclosures.
Thus, combustion enclosures, such as the combustion chambers and nozzles of rocket engines, in particular engines using liquid propellants, have walls which are in contact with combustion gases that constitute a high temperature medium, and such walls are generally cooled while they are in operation.
A common cooling technique consists in providing the walls of such enclosures with cooling channels. This applies in satellite launchers and space planes, and also in satellite thrusters, nuclear reactors, and high efficiency boilers, and it can also apply to heat shields or to the nose cones of vehicles traveling at very high speed.
Specifically in the context of rocket engines, various methods have already been proposed for manufacturing the walls of combustion chambers to enable cooling channels extending in a longitudinal direction to be included therein, which channels convey a cooling fluid that may be one of the propellant components used for feeding the rocket engine, so that the cooling system thus constitutes a regenerative system.
The techniques for manufacturing such combustion chambers are nevertheless difficult to implement, lengthy, and expensive.
In certain particular applications, it is also useful to be able to heat up an enclosure that is cold, by causing a hot fluid to circulate via passages formed in the wall of the enclosure, which thus also constitutes a regenerative circuit.
In a first technique for manufacturing regeneratively-cooled combustion chambers for liquid propellant rocket engines, cooling channels are machined in an inner base body formed as a single piece of a metal that is a good conductor of heat, such as copper. The cooling channels are thus separated from one another by partitions of the base body, and an outer cover is made by electrodeposition of multiple layers of nickel alternating with machining corrections that are necessary between each of the electrodeposition passes. The channels are closed prior to electrodeposition by applying a conductive resin.
FIG. 14 shows an example of a combustion chamber wall made using that technique.
An inner jacket 104 that is made by forging, e.g. out of a metal material such as Narloy Z, has cooling channels 105 that are made by machining.
A layer 107 for closing the channels 105 is made by electrodeposition and is itself covered in nickel that is likewise deposited by electrodeposition. Various elements of the outer shell 109 made of a superalloy such as Inconel-718, for example, are assembled together via joins 110 by means of electron beam welding.
The operations of forming the inner jacket 104 of the combustion chamber and of closing the channels 105 by electrodeposition constitute major drawbacks of that method. Those operations are lengthy and expensive. Furthermore, each of the welds 110 used for final assembly of the components of the chamber constitutes a potential risk of breakage. In a second prior art technique of combustion chamber manufacture, attempts have been made to eliminate those disadvantages by using the plasma-forming methods.
FIG. 15 shows an example of a combustion chamber wall made using that second manufacturing technique which consists in making all or part of the structure of the combustion chamber by thermally spraying powders of defined alloys.
In an example of such a method in which the wall of the combustion chamber is made starting from the inside and going towards the outside of the chamber, a spraying core 1 is made out of mild steel machined to the inside dimensions of the combustion chamber that is to be obtained.
First spraying under a partial vacuum then serves to use copper alloy (Narloy Z, . . . ) to make the jacket 4 of the future chamber on the surface of the core 1. The following operation consists in machining the cooling channels 5 and in inserting a consumable filler material therein. After excess filler has been removed by machining, a second operation of spraying the copper alloy under partial vacuum enables a layer 7 to be formed for closing the channels. Immediately thereafter, the superalloy shell 8 is built up directly on the copper jacket by thermal spraying. The final operation consists in chemically eliminating the filler material so as to open up the channels 5, and also so as to remove the spraying core 1.
Proposals have also been made to provide a support core built up from a plurality of parts, thus making it possible to reuse the core. By way of example, a core can be constituted by two stainless steel cones that are separated from each other by a washer of mild steel, with the assembly being covered in a deposit of steel. Once the combustion chamber has been made, the dissolving of the inserts placed in the cooling channels is accompanied by dissolving the washer and the steel deposit. The two cones can then be removed and recovered.
Nevertheless, known methods remain relatively unsatisfactory, in particular because of the slowness of the process whereby the inserts are dissolved and the temporary layers are eliminated, and because of difficulties in making structures that are of large dimensions in satisfactory manner.
The object of the invention is to remedy the above-specified drawbacks and to enable regenerative circuit structures to be manufactured in a manner that is more convenient than in prior art methods while also making it possible to optimize the characteristics of the manufactured structures, even when the structures are of large dimensions and are subjected to high heat flux densities.
According to the invention, these objects are achieved by a method of manufacturing a high heat flux regenerative circuit comprising a structure having an inner functional surface in contact with a first fluid and a set of channels formed in the body of the structure for conveying a second fluid in heat exchange relationship with the first fluid, the method consisting in using thermal spraying and machining operations around a reusable support core to build up the structure from said inner functional surface, the method being characterized in that it comprises the following steps:
a) placing a support core representing the inner profile of the structure about an axis of rotation, the support core being made of a material whose coefficient of thermal expansion is very close to or slightly greater than that of the body material of the structure;
b) making an intermediate layer on the support core out of a material that is different from that of the support core and that of the body of the structure;
c) forming a series of channels regularly spaced apart around the core and opening out to face said intermediate layer, each of the channels being provided with a soluble insert comprising a mixture of organic binder and metal powder;
d) preheating the support core to a temperature greater than about 850xc2x0 C. and making the body of the structure by thermal spraying under a vacuum or low pressure by means of a plasma torch, while maintaining the temperature of the support core at said temperature greater than 850xc2x0 C.;
e) without dismantling the support core, machining channels in the form of grooves in the outside of the body of the structure;
f) filling the channels in the body of the structure with soluble inserts comprising a mixture of organic binder and metal powder;
g) forming a layer for closing the channels in the body of the structure and forming an outer envelope of the structure by thermal spraying under a vacuum or low pressure by means of a plasma torch, after preheating and while maintaining the support core at a temperature greater than about 850xc2x0 C.;
h) eliminating the soluble inserts in the channels of the body of the structure, the soluble inserts in the channels formed around the support core, and the intermediate layer; and
i) withdrawing the reusable support core.
According to a preferred characteristic, prior to the step of filling the channels machined in the body of the structure or around the support core, filament of tubular inserts, preferably made of a plastics material such as a polyamide resin for example, are inserted in the bottoms of the grooves forming the channels so as to make it possible subsequently to form cavities in the bottoms of the channels beneath the soluble inserts. The filamentary or tubular inserts are withdrawn prior to the following thermal spraying step.
In a particular embodiment, the step of filling the channels in the body of the structure with soluble inserts consists in an operation of filling the channels incompletely followed by a step of thermally spraying a metal material which finishes off the filling of the channels to form a metal layer which also covers the ribs between the channels, and then followed by a step of machining the surface of said metal layer until the free tops of said ribs have been laid bare.
When manufacturing structures of large dimensions, during the operations of thermal spraying with preheating of the support core to a temperature greater than about 850xc2x0 C., additional heat is provided throughout the thermal spraying operation by means of an additional heater device close to the support core.
According to particular characteristics of the invention:
the body of the structure is made by thermally spraying a Cuxe2x80x94Agxe2x80x94Zr alloy powder;
the support core is made of pure copper;
the intermediate layer is made by thermally spraying iron powder;
the filamentary inserts are constituted by filaments based on polyamide resin;
the step of thermally spraying a metal material to terminate filling of the channels consists in thermally spraying iron powder under a vacuum or under low pressure by means of a plasma torch;
the step of forming a layer for closing the channels in the body of the structure consists in thermally spraying a Cuxe2x80x94Agxe2x80x94Zr alloy powder, and the step of forming an outer envelope consists in thermally spraying an alloy powder based on nickel such as a nickel-copper based alloy constituted by MONEL K500 or NU30AT; and
the step of eliminating the soluble inserts from the channels in the body of the structure or from channels formed around the core is performed by circulating a fluid such as hydrochloric acid.
The method may include an additional step of forming a layer of porous copper by thermal spraying performed between forming the layer for closing the channels in the body of the structure and the step of forming an outer envelope.
In a first advantageous embodiment of the invention, step c) of forming a series of channels that are regularly distributed around the core takes place prior to step b) of making an intermediate layer and comprises machining the support core from the outside after it has been mounted on said axis of rotation to obtain channels in the form of grooves, and filling said channels in the support core by means of soluble inserts comprising a mixture of organic binder and metal powder.
In which case, the method may further include, after the step of making an intermediate layer, an additional step consisting in making a layer of low roughness by thermal spraying under a vacuum or under low pressure by means of a plasma torch, which layer of low roughness is made of a thermal barrier forming material of the metal type or of the oxide type.
By way of example, the thermal barrier may be made by thermally spraying a powder of a superalloy such as MCrAlYTa.
Advantageously, the thermal barrier is made, prior to spraying the superalloy, by spraying yttrium-containing zirconia powder so as to form a surface layer of said yttrium-containing zirconia material.
In another particular embodiment, step c) of forming a series of channels regularly distributed around the core takes place after step b) of making an intermediate layer, and comprises depositing a copper-based alloy on the intermediate layer, machining the copper-based alloy to define radial fins between which gaps are formed to constitute said channels, and filling said channels with soluble inserts comprising a mixture of organic binder and metal powder.
In which case, the method may further include a step consisting in depositing an additional iron layer to fill portions of the channels that have remained empty over the soluble insert and also to cover the fins, and a step consisting in machining said additional iron layer to cause the copper-based fins to emerge so as to conserve only a thin layer of iron over each soluble insert, said layer being flush with the free faces of the fins.
This second embodiment leads to a structure for the regenerative circuit of the finned heat exchanger type.
As a variant embodiment it is possible to achieve a structure of a regenerative circuit of the finned heat exchanger type within the frame of the first embodiment wherein step c) of forming a series of channels regularly distributed around the core takes place before step b) of making an intermediate layer, the channels regularly disytributed around the core being machined in the core from the outside.
According to this variant embodiment, the method further includes, after step b) of making an intermediate layer on the core, a further step b1) consisting in machining in the intermediate layer, outer grooves having a height h and a width l such that h/lxe2x89xa61, to form radial fins during the step of making the body of the structure by thermal spraying.
Thus this variant of the first embodiment of the method according to the invention allows to make radial fins of small height which avoid the need of forming on the intermediate layer a thermal barrier by spraying a layer of material, but the fins do not delimit main channels opening out to face the intermediate layer. The main channels are formed by machining in the core from the outside.
The method of the invention can be applied in general manner to manufacturing a structure constituted by a heat exchanger.
The method of the invention is particularly adapted to manufacturing a structure constituted by a high heat flux combustion enclosure such as a combustion chamber and a nozzle of a rocket engine, in particular a high power rocket engine of the cryogenic type.
The invention also provides regenerative circuits obtained by the various implementations of the above-described method of manufacture.